This invention relates to seal assemblies of the type including air flow past the seals to prevent oil or contaminants from moving in a direction opposite to such air flow. In particular, this invention relates to an improved lip seal which includes a bellows or flexible boot portion enabling movement of the seal in a radially outward direction.
Internal combustion engines require efficient seals at the ends of the crankshaft. These sealing means, in the form of circumscribing seals, perform several functions. First, they retain oil or other lubricant in the engine's crankcase. Secondly, they prevent contaminants from entering the engine from the surrounding environment. Such contaminants can, of course, result in shortened engine life.
One type of seal that has been found to be effective in this environment is that shown in U.S. Pat. No. 3,575,426 assigned to the assignee hereof. With this patent device, a pair of lip seal assemblies are coaxially mounted around a rotational shaft to form a chamber therebetween adapted to be pressurized by air which lifts the seal out of contact with the shaft. In this manner, continuous streams of air will flow past the seals to prevent oil or contaminants from moving in a direction opposite to such air flow. In addition, seal life is enhanced because seals do not normally run against the surface of the shaft. However, when the shaft is at rest the seal is forced to positively contact the shaft by means of retainer springs located radially outwardly of the lip. However, the subject seal tends to be rather stiff and lacks the flexibility desired. This results in some control problems being engendered in the selection of the amount of air flow necessary to lift the lip from the shaft surface.
The lip-type seal thus described commonly has a service life of up to 4,000 hours, but is limited to about 4,500 ft/min. surface speeds and 250.degree. F. oil sump temperatures. In addition, failure rates are high, primarily due to wear and installation problems.